BRIDGE REPAIR/REHABILITATION FEASIBILITY STUDY

process should not pose a significant concern for larger members that do not currently exhibit
decay.)
The required saturation of the surfaces of the timber members for in-place surface treatments
introduces concerns of run off of the toxic chemicals. Implementation of fully effective
containment that ensures that spilled chemicals will not enter the Mitchell River will be a
challenge due to the limited clearance beneath the bridge and the limited available space between
the pile bents. As such, it may be difficult to obtain a permit for this work. With these concerns
and risks, in-place surface treatment would not be a prudent alternative for the timber stringers.
Alternatively, in order to increase the service life of the timber and eliminate the above concerns
and risks associated with in-place preservative treatments, the timber stringers and blocking
should be replaced.
Rehabilitation Scope: In addition to the work addressed in the Repair Scope, the bascule span
stringers should be strengthened or replaced with larger members. One method of strengthening
includes attachment of fiber reinforced polymer (FRP) to the sides of the timber members.
Larger members will increase the weight of the bascule span and thus the counterweight will
need to be replaced with a heavier counterweight. The steel counterweight box will need to be
replaced and a new counterweight box that contains a greater proportion of steel ballast than
concrete ballast. If the weight required to balance the span is too great, it may be required to
provide material with greater density in the counterweight, such as lead or an all steel
counterweight. Alternatively, an all steel counterweight can be used with stacks of stainless steel
plate (see 4.3.5 below.)
As noted above, it may be necessary to modify the layout of the timber stringers in order to
accommodate mounting of crash tested timber traffic railing (see 4.2.5 above) and to
accommodate a shift in the location of the sheave poles (see 4.3.4 below.)
Functionality and Safety: The stringers have no significant impact on functionality and safety.
Load Capacity: As noted above, the 1997 load rating analysis identified that the shear capacity of
the bascule span timber stringers are less than the Inventory Level capacities. The shear capacity
of the members would need to be increased approximately 20% in order to provide the required
capacity. With the substandard capacity, these structural members may not have adequate
capacity to carry current loading indefinitely and that degradation of the elements under loading
is eventually expected. In accordance with FHWA guidelines, if the existing bridge was to
remain and was rehabilitated, a design exception would need to be granted to allow the bascule
span members to remain or the substandard members would need to be strengthened or replaced
with stronger members.
The impact damage to the underside of the bascule span stringers does not significantly affect the
load capacity of the members.
Repair/Rehab. Feasibility Study March 10, 2011
Bridge No. C-07-001 (437) 25 Final Report